It is well known that, besides proper burdening of materials, the geometrical distribution of charge material in a blast furnace has a decisive influence on the hot metal production process since it determines among others the gas distribution. In order to achieve a desired distribution profile in view of an optimal process, two basic aspects are of importance. Firstly, material is to be directed to the appropriate geometric locus on the stock-line for achieving a desired pattern, typically a series of closed concentric rings or a spiral. Secondly, the appropriate amount of charge material per unit surface is to be charged over the pattern.
Regarding the first aspect, geometrically well-targeted distribution can be achieved using a top charging installation equipped with a distribution chute that is rotatable about the furnace axis and pivotable about an axis perpendicular to the rotational axis. During the last decades, this type of charging installation commonly referred to as BELL LESS TOP™ has found widespread use throughout the industry among others because it allows directing charge material accurately to any point of the stock-line by appropriate adjustment of the chute rotation and pivoting angles. An early example of such a charging installation is disclosed in U.S. Pat. No. 3,693,812 assigned to PAUL WURTH. In practice, this kind of installation is used to discharge cyclically recurring sequences of charge material batches into the furnace by means of the distribution chute. The distribution chute is typically fed from one or more top hoppers (also called material hoppers) arranged at the furnace top upstream of the chute, which provide intermediate storage for each batch and serve as a furnace gas sluice.
In view of the second aspect, i.e. controlling the amount of material charged per unit surface area, the above-mentioned type of charging installation is commonly equipped with a respective flow control valve (also called material gate) for each top hopper, e.g. according to U.S. Pat. No. 4,074,835. The flow control valve is used for adjusting the flow rate of charge material discharged from the respective hopper into the furnace via the distribution chute to obtain the appropriate amount of charge material per unit surface by means of a variable valve opening.
Flow rate adjustment usually aims at obtaining a diametrically symmetrical and circumferentially uniform weight distribution over the desired pattern, which typically requires a constant flow rate. Another important aim is to synchronize the end of a batch discharge with respect to the end of the pattern described by the distribution chute. Otherwise, the hopper may be emptied before the chute reaches the end of the pattern (“undershoot”) or there may remain material to be discharged after the pattern has been fully described by the chute (“overshoot”).
Japanese patent applications JP 04 198412, JP 56 047506 and JP 59 229407 propose methods that aim at avoiding undershoot or overshoot. In each of these methods, the valve opening of the flow control valve is fixed during the discharge of a given batch but readjusted for a subsequent discharge in case overshoot or undershoot has occurred. As an alternative to readjusting the valve opening, JP 56 047506 also suggests varying the rotational sped of the distribution chute while maintaining an unchanged valve opening. As will be understood, whilst addressing the problem of undershoot or overshoot, the methods proposed in JP 04 198412, JP 56 047506 and JP 59 229407 do not warrant a constant flow rate required for circumferentially uniform weight distribution over the desired pattern. In fact, with a valve opening that remains constant during the discharge of a given batch, the flow rate inevitably varies during the discharge among others because of the decreasing residual mass that remains in the hopper.
In other known approaches, the valve opening is therefore varied during the time of discharge of a given batch. In a typical approach of this kind, the flow control valve is initially set to a predetermined “average” position i.e. “average” valve opening corresponding to an average flow rate. In practice, the average flow rate is determined in function of the initial volume of the batch stored in the respective top hopper and the time required by the distribution chute for completely describing the desired pattern. The corresponding valve opening is normally derived from one of a set of pre-determined theoretical valve characteristics for different types of material, especially from curves plotting flow rate against valve opening for different types of material. As discussed e.g. in European patent no. EP 0 204 935 a valve characteristic for a given type of material and a given valve may be obtained by experiment. EP 0 204 935 proposes regulating the flow rate by means of “on-line” feedback control during the discharge of a batch in function of the monitored residual weight or weight change of charge material in the discharging top hopper. In contrast to earlier U.S. Pat. Nos. 4,074,816 and 3,929,240, EP 0 204 935 proposes a method which, starting with a predetermined average valve opening, increases the valve opening in case of insufficient flow rate but does not reduce the valve opening in case of excessive flow rate. EP 0 204 935 also proposes updating data indicating the valve position required to ensure a certain output of a particular type of material, i.e. the valve characteristic for a particular type of material, in the light of results obtained from previous charging.
Japanese patent application JP 2005 206848 discloses another method of “on-line” feedback control of the valve opening during the time of discharge of a batch. According to JP 2005 206848, the valve opening is readjusted by means of “dynamic control”, which uses integral and proportional control action, in discrete steps or intervals. Each interval corresponds to a full revolution of the rotating distribution chute during the discharge. This on-line “dynamic control” readjusts the valve opening for a subsequent interval during the discharge in function of residual weight to be discharged and remaining discharge time. In addition, JP 2005 206848 proposes applying two calculations, a “feed forward” correction and a “feed back” correction, to determine more accurately the required initial valve opening for the first discharge interval i.e. the first chute revolution.
European patent EP 0 488 318, discloses another method of flow rate regulation by means of real time control of the degree of opening of the flow control valve and also suggests the use of tables that represent the relationship between the degree of opening and the flow rate according to different kinds of material akin to the above-mentioned valve characteristic. EP 0 488 318 proposes a method aiming at obtaining a constant ratio of flow rate to (average) grain diameter during the discharge in view of achieving a more uniform gas flow distribution.
The practice of “on-line” flow regulation according to EP 0 204 935 is currently widespread. Despite its obvious benefits regarding circumferentially uniform weight distribution, this approach leaves room for improvement. For instance, it is deemed not sufficiently adaptive to a wider variety of batch properties, e.g. to batches consisting of a mixture of different charge materials, or to a wider variety of operating conditions of the top charging installation. Moreover, known approaches of “on-line” feedback control, e.g. according to EP 0 204 935 or JP 2005 206848, require accurate selection and tuning of the control parameters to achieve good results.